Governments mobilize radiation detectors to stop the illicit movement of nuclear material such as plutonium and uranium. Previous approaches to neutron detection have relied upon an isotope of helium gas, helium 3 or 3He, a limited resource generated during the construction and/or decommissioning of nuclear weapons which is already showing signs of a global short supply. Due to increasing 3He shortages and the resulting increase in associated costs, neutron detectors utilizing 3He cannot be economically deployed at scales. Efforts to develop replacement technologies have been initiated however, none of these efforts have produced a cost effective, scalable solution.
The lack of scalable technology has limited the evolution of existing systems to meet evolving threats. Specifically, current modeling efforts show that the deployment of a large, networked array of detection technologies—where the detectors are placed at potential points of attack, material source locations, and discreetly at randomized points of transportation pathways-will lead to the greatest increase of overall security against nuclear threats.
Plutonium and highly enriched uranium (HEU)—materials that can be used in a nuclear weapon—emit both gamma rays and neutrons. After the attacks on Sep. 11, 2001, the U.S. government sought to strengthen border defenses against smuggled Special Nuclear Materials (SNM). To detect SNM, Federal, state, and local governments deployed detection units using on 3He gas in proportional counters wrapped in high-density polyethylene (HDPE)—a technology pulled from physics laboratories and the nuclear power industry. Polyvinyltoluene (PVT) plastics hooked up to photomultiplier tubes (PMT), pulled from the scrap-steel industry, were used to detect gamma rays emitted by HEU, as well as other dangerous radioactive sources that could be used to create a radiological dispersive device. Handheld devices, which have better gamma ray energy resolution than PVT, supported the main scanning capabilities of these larger 3He and PVT detectors.
This initial detection capability had challenges. The initial deployment of neutron detectors severely depleted the limited stockpile of 3He, driving costs sky-high and limiting scalability of deployment. Equally problematic were the number of false positive alarms that were due to the poor energy resolution of PVT, increasing overall scanning times and limited the usability of the systems. Multiple government R&D programs over the past ten years have invested in 3He alternatives for neutron detection, as well as improved energy resolution gamma ray detection units. However, while some alternative materials have emerged, none of the R&D programs succeeded in reducing the cost of these systems. Furthermore, low-cost, large-surface-area gamma ray detectors have not been realized at any reasonable price point, with most of the work still focusing on smaller surface area (around 200 cm2 active regions) devices. The original desire to replace neutron detectors with large surface high-energy-resolution gamma-ray detection (which could detect the gamma ray signals from both Pu and HEU) has therefore faded. Given that 1.2 million kilograms of Pu has been produced since World War II, and its key signature is neutron emission, neutron detection is now considered a non-negotiable component of threat detection capability.
For these reasons, new neutron detection solutions are needed. The solution should:                Be low cost and independent of 3He. This will enable scalable, affordable solutions;        Have low probability for gamma-ray induced false positives by having high gamma ray rejection and secondary gamma ray isotope identification system;        Be rugged and long lived for compatibility with military CONOPS; and        Hit metrics of capture area and efficiency to detect the desired threats will be a major advance in the overall reduction of nuclear threats.        
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications and variations thereof will be apparent to those skilled in the art.